Rheological Characterization of Recycled Polyolefin Compounds
Single-point melt index fails to predict processability; high-shear capillary and dynamic oscillatory testing prevent costly tooling scrap and cycle delays.
Rabinowitsch-Mooney Correction
Meaning
Rheological adjustment methods account for non-Newtonian flow behavior in capillary viscometry by correcting the shear rate calculated at the wall. The rabinowitsch-mooney correction applies to polymer melts and solutions exhibiting pseudoplastic properties, where the velocity profile across a tube deviates from the parabolic shape assumed for purely Newtonian fluids. By incorporating the flow index determined from the slope of the logarithmic plot of shear stress against apparent shear rate, the calculation yields the true shear rate at the wall.
This mathematical refinement ensures that viscosity measurements remain accurate despite the inherent shear thinning characteristics of the material.
Rheological Adjustment
Extrusion and capillary rheometry require this treatment to derive reliable data from raw pressure and flow rate readings. The rabinowitsch-mooney correction converts the apparent shear rate to the actual shear rate by multiplying the former by a factor derived from the flow behavior index of the resin. Without this adjustment, the calculated viscosity will appear falsely high, masking the true processability of the polymer.
Technicians use this data to predict how a resin will behave during high-shear operations like injection molding or die extrusion. Reliable measurements depend on this conversion to separate equipment effects from intrinsic material properties.
Material Specification
Datasheets for engineering resins typically state viscosity values processed through these correction formulas to allow for legitimate comparisons between suppliers. A polymer supplier provides a value that reflects the behavior of the material under specific, corrected conditions, whereas a processor may observe a different value if the test methodology ignores the wall effect. Moulders rely on these corrected figures to scale up processes from laboratory analysis to production machinery.
Discrepancies between the datasheet and shop floor results frequently arise when the influence of shear rate on chain orientation remains unaccounted for in the equipment settings.
Process Variable
Accurate calibration of injection molding cycles prevents defects such as high internal stress, warpage, or short shots that occur when the operator misjudges the effective viscosity of the melt. Excessive pressure required to fill a cavity often stems from a lack of awareness regarding how the shear rate changes throughout the flow channel. If the control system operates on uncorrected assumptions, the cooling times and packing pressures become difficult to stabilize across different batches of virgin material.
Proper application of these formulas allows the moulding facility to maintain consistent part quality despite minor variations in the feedstock. Stability in the production run relies on identifying the relationship between shear stress and strain rate throughout the flow path.